The overall purpose of this research is to study the biology of adenosine in the heart particularly in relation to the modulation of ventricular function. The objective is to investigate the potential for endogenous adenosine, acting through specific cell surface receptors, to modulate myocardial adenylate cyclase activity and thereby modify the inotropic responsiveness of the heart to selected agents in both the normal and the pathophysiologic state. Complementary studies will be performed with intact hearts and with an enriched preparation of cardiac sarcolemma. Experiments are proposed to: 1) Identify and characterize adenosine binding sites on cardiac sarcolemma. Radioligand binding studies will be performed using H3 phenylisopropyladenosine and H3 - adenosine 5' -N-ethylcarboxamide as ligands. 2) Define the effects of adenosine and adenosine analogues on basal and isoproterenol-activated adenylate cyclase activity. The concentration of GTP, monovalent cations and divalent cations as well as endogenous adenosine will be controlled to provide an accurate assessment of the concentrations of adenosine required to influence myocardial adenylate cyclase. 3) Determine if the membrane sites identified by radioligand binding represent specific cell surface adenosine receptors which mediate effects of the nucleoside on adenylate cyclase activity. 4) Determine the effects of prolonged myocardial exposure to adenosine on the inotropic responsiveness of the heart to selected agents and on the membrane receptor-adenylate cyclase systems. In preliminary studies with the perfused guinea pig heart, adenosine has been found to produce a persistent desensitization of the myocardium to the inotropic effects of isoproterenol. 5) Evaluate the nature of adenosine receptors and receptor-adenylate cyclase mechanisms in the failing heart where elevated levels of adenosine are observed. These experiments will be conducted with sarcolemma isolated from the hearts of dogs in congestive failure. The results from these studies will help clarify the potential for adenosine to play a role in the modulation of adenylate cyclase and the ventricular contractile response of the normal and failing myocardium.